1. Field of the Invention
The present invention relates to an energy trap piezoelectric resonator used for, for example, a piezoelectric oscillator, and, more particularly, to an energy trap piezoelectric resonator making use of a harmonic wave in a thickness longitudinal vibration mode.
2. Description of the Related Art
Previously, higher frequencies in electronic devices have caused various energy trap resonators making use of a harmonic wave in a thickness longitudinal vibration mode to be proposed.
For example, Japanese Unexamined Patent Application Publication No. 4-216208 discloses a piezoelectric resonator shown in FIG. 10.
A piezoelectric resonator 101 includes a rectangular plate piezoelectric substrate 102. The piezoelectric substrate 102 is formed of piezoelectric ceramics polarized in a thickness direction. A first excitation electrode 103 is provided at the center of an upper surface of the piezoelectric substrate 102, and a second excitation electrode 104 is provided at the center of a lower surface of the piezoelectric substrate 102 so as to oppose the excitation electrode 103. A portion where the excitation electrodes 103 and 104 oppose each other is a piezoelectric vibration portion.
The excitation electrodes 103 and 104 are electrically connected to extraction electrodes 105 and 106, respectively. In the piezoelectric resonator 101, a third harmonic wave in a thickness longitudinal vibration mode is used. Therefore, since a fundamental wave in a thickness longitudinal vibration mode becomes spurious, it is desirable to suppress the fundamental wave. Consequently, in the piezoelectric resonator 101, partial electrodes 107 and 108 are provided on the upper surface of the piezoelectric substrate 101 so as to extend along side edges 102a and 102b of the piezoelectric substrate 102, respectively. Partial electrodes 109 and 110 are also provided on the lower surface so as to extend along respective side edges.
In the piezoelectric resonator 101, when the fundamental wave is being transmitted from the piezoelectric vibration portion to a surrounding area, and the partial electrodes 107 to 110 are provided, the fundamental wave is suppressed by a piezoelectric short-circuit effect and mechanical loads of the partial electrodes 107 to 110. In other words, the document states that the fundamental wave can be suppressed by making use of mass loading of the partial electrodes 107 to 110.
Japanese Unexamined Patent Application Publication No. 11-177375 discloses a piezoelectric resonator shown in FIG. 11. In a piezoelectric resonator 151, a first excitation electrode 153 is provided at an upper surface of a rectangular plate piezoelectric substrate 152, and a second excitation electrode 154 is provided at a lower surface of the piezoelectric substrate 152. A portion where the excitation electrodes 153 and 154 oppose each other with the piezoelectric substrate 152 disposed therebetween is an energy trap piezoelectric vibration portion. This document states that the piezoelectric substrate 152 is polarized in the thickness direction and that a third harmonic wave in a thickness longitudinal vibration mode is used. The excitation electrodes 153 and 154 are provided consecutively with extraction electrodes 155 and 156, respectively. The extraction electrode 156 is connected to a mounting electrode 158 through an end surface of the piezoelectric substrate 152.
In the piezoelectric resonator 151, a floating electrode 157 is provided on the upper surface of the piezoelectric substrate 152 so as to be situated at a side opposite to the side towards which the excitation electrode 153 is extended with respect to the extraction electrode 155. Here, a fundamental wave which is transmitted from the piezoelectric vibration portion to a surrounding area is suppressed in a thickness longitudinal vibration mode by mass loading of the floating electrode 157, so that a resonance characteristic in which the third harmonic wave in a thickness longitudinal vibration mode can be effectively used.
As stated in Japanese Unexamined Patent Application Publication No. 4-216208 and Japanese Unexamined Patent Application Publication No. 11-177375, various structures making use of mass loading of metallic materials of which the electrodes are made have been proposed in order to suppress the fundamental wave in a thickness longitudinal vibration mode. In other words, since the fundamental wave becomes spurious when a harmonic wave in a thickness longitudinal vibration mode is used, there has been a strong demand for suppressing the fundamental wave. In order to suppress the fundamental wave by mass loading, an attempt has been made to dispose the partial electrodes 107 to 110 or floating electrode 157 around the piezoelectric vibration portion.
However, in the related structures which try to suppress the fundamental wave by mass loading, it is difficult to sufficiently suppress response by the fundamental wave when making use of a harmonic wave in a thickness longitudinal vibration mode. In addition, when the fundamental wave is sufficiently suppressed by a large mass load, the response of the harmonic wave in a thickness longitudinal vibration mode tends to be suppressed too.